1. Field of the Invention
The present invention pertains to a method and apparatus for analyzing effluent streams from various process tools used to make semiconductors, and in particular, to an on-line UV-Visible analyzer system for measuring the concentrations of homonuclear diatomic halogens (F2, Cl2, Br2, and I2) in a gas flow stream, and methods for using the analyzer system.
2. Description of the Related Art
In the manufacture of integrated circuits (IC), sequences of thin film deposition and etching steps are performed in order to construct several complete electrical circuits (chips) on monolithic substrate wafers. The general principles of IC manufacturing are described in a publication entitled Handbook of Semiconductor Manufacturing Technology, Y. Nishi and R. Doery editors, Marcel Dekker, New York, N.Y. 2000.
In a typical manufacturing sequence, molecular gases containing halogen atoms are often used in processes to remove materials, either from an integrated circuit substrate or, from the internal components of deposition equipment. The process that removes material from integrated circuit substrates is typically referred to as etching, while the process for removing deposits on the inner walls of the deposition tools is called chamber cleaning. Chamber cleaning is necessary to maintain the quality of the film produced in the deposition processes. Etching is necessary to produce the desired circuit structure on the substrates.
Molecular gases containing halogens are often used in film removal processes because reactions with certain critical substrate materials form energetically stable, gaseous byproducts. These byproducts evolve from the substrate or process tool surface. They are then easily removed from the process equipment by vacuum pumping. In a typical process step, silicon dioxide (SiO2) is deposited as an electrical insulating layer on the surface of a silicon wafer for example, by a plasma enhanced chemical vapor deposition (PECVD) process. Other thermal deposition processes are known to produce semiconductor films. After removal of the wafer from the process chamber, residual SiO2 remains on the inside of the process chamber and must be removed to prevent the formation of particles. In some processes, a gas containing fluorine such as NF3 or C2F6 is converted in an electrical discharge plasma to atomic fluorine, according to the reaction: 
This plasma can be generated between electrodes located in the deposition chamber. In this case, the process is termed xe2x80x9cin situ plasma chamber cleaningxe2x80x9d. The plasma can also be generated upstream of the process chamber in which case it is termed xe2x80x9cremote plasma downstream chamber cleaningxe2x80x9d.
Atomic fluorine reacts with SiO2 to form volatile byproducts SiF4 and O2, according to the reaction: 
A competing process is the recombination of atomic fluorine radicals to form molecular fluorine by the reaction:
2F.xe2x86x92F2(g)xe2x80x83xe2x80x83(3)
In most processes, the rate of reaction (1) is constant. As SiO2 is removed from the process chamber, the rate of reaction (2) slows. This results in an increase in the number density of atomic fluorine in the process chamber and a subsequent increase in the rate of reaction (3). The time when the residual SiO2 is completely removed from the internal components of the process chamber is called the xe2x80x9cendpointxe2x80x9d. The endpoint is marked by a plateau in the atomic fluorine (F.) number density and in the molecular fluorine (F2) emission rate. Process control and optimization requires accurate determination of the endpoint to terminate the cleaning process.
For in situ plasma cleaning processes, an electrical discharge exists in the vicinity of the residual film being removed. There is an emission of light from the discharge at wavelengths specific to gases present in the discharge. One method for determining the endpoint of an in situ cleaning process is to monitor light emission from atomic fluorine. At the endpoint, this signal increases to a constant value due to the increase in fluorine concentration within the process chamber.
In remote plasma cleaning, the plasma is formed upstream of the process chamber containing the residual film. By the time the radical species flow into the process chamber, optical emission from the reactant gases ceases. Therefore, optical emission cannot be used to determine the endpoint.
Another method for determining the endpoint is to monitor the concentrations or flow rates of product species in the gas effluent from the process tool. In U.S. Pat. No. 6,079,426 patentees disclose a method to determine the endpoint by monitoring either the change in pressure in the process tool when the exhaust capacity is fixed, or the degree that the exhaust pump must be throttled to maintain a constant pressure. In other words, this method monitors the changes in the exhaust pump throughput. This method is only applicable to processes in which the number of atoms or molecules generated in the process changes by a measurable amount as endpoint is reached. Unfortunately, most of the chamber cleaning procedures use large amounts of inert diluent gases (e.g. Helium) in addition to reactive gases, hence the changes in the number of atoms or molecules generated by the plasma processes are greatly diluted by the large inert gas flow and may become too small to provide an accurate indicator of the end point.
Methods that directly monitor chemical species within the process effluent are more desirable from a process control perspective.
In U.S. Pat. No. 5,812,403 patentees disclose an infrared absorption endpoint monitor based on the absorption of infrared light by species present within the exhaust gas of a process tool. This endpoint detection method is specific to processes that form species that absorb infrared light in a specific band of infrared wavelengths. It is highly desirable to have a widely applicable system that can detect the endpoint following chemical vapor deposition of any material in which a molecular species containing a halogen is used as the reactant. A large number of deposition residues are removed by reactions with halogen-containing gases. These include tungsten, silicon, silicon dioxide, and silicon nitride. It is also desirable to have a system that is independent of the energy source used in the chamber cleaning process.
An IC process effluent stream may contain one or more homonuclear diatomic halogen gases. For example, NF3, and C2F6 based chamber cleaning processes emit F2 in the process effluent stream. Use of ClF3 in chamber cleaning emits both Cl2 and F2. Use of fluorocarbons such as C4F8 and C4F6 etc. in dielectric plasma etching may emit F2. Use of HBr and BCl3 in conductor (for example, polysilicon and metals) plasma etching leads to emission of Cl2 and Br2. These homonuclear diatomic halogen gases (such as F2, Cl2, Br2, and I2) are highly toxic, reactive, and corrosive. The amounts of these halogen gases released from a semiconductor fabrication site cannot exceed government mandated limits. Therefore, homonuclear diatomic halogen gases must be quantified prior to release to the environment or an abatement system.
Typical instruments used to measure concentrations of effluent species in real-time include infrared and mass spectrometers. Both techniques have severe limitations in this application.
In U.S. Pat. No. 6,154,284, patentees disclose use of a tunable diode laser absorption spectrometer (TDLAS) to quantify species in an IC process effluent stream. TDLAS is a special kind of infrared spectrometer. Other types of infrared spectrometers include non-dispersive infrared (NDIR) and Fourier Transform Infrared (FTIR) types.
Infrared spectrometers determine the concentration of various species in a gas cell by measuring the decrease in the intensity of infrared radiation traversing the cell due to absorption. The degree of absorption is dependent on the concentration of each absorbing species. The pattern of infrared absorption as a function of wavelength, or xe2x80x9cspectrumxe2x80x9d is unique for each absorbing species. Absorption of infrared light generally results from the excitation of specific vibrational modes in the absorbing molecule. Not all vibrational modes, however, can be excited by infrared light. Those that do not absorb light are xe2x80x9cinfrared inactivexe2x80x9d. The single vibrational mode of homonuclear diatomic molecules, such as N2, O2, F2, Cl2, and Br2 are infrared inactive. As a result, none of the homonuclear diatomic halogen molecules absorbs infrared radiation. Thus, they cannot be quantified using infrared spectroscopy.
Mass spectrometers determine the concentrations of various species in a gas sample by ionizing the sample by collisions with high energy (70 eV) electrons, then separating and detecting fragment ions. The pattern that an analyte fragments into is unique for most molecular species. The ion current for each fragment is dependent on the concentration of the analyte. Thus the concentration of the analytes can be determined from the fragment ion currents.
Mass spectrometry is typically performed in a high vacuum environment, preferably at pressures less than 10-6 Torr. Since the ionization and separation processes must be done in regions where the probability of collisions with other molecules over the measurement time is low, the vacuum pumps required to reduce the pressure to this level are large and require substantial time to set up. Due to their size, mass spectrometers with the required sensitivity are expensive to transport.
The sensitivity of the detector used in mass spectrometers is typically not stable over long periods of time. This requires that mass spectrometers be calibrated frequently to accurately measure the fragment ion current as a function of analyte concentration.
Halogen molecules are highly corrosive species. The presence of these species near the working components of mass spectrometers such as ion sources, detectors, and pumps causes their performance to degrade and eventually these devices cease to operate. These high performance components are expensive to refurbish or replace and leads to a higher cost of ownership for the analyzer.
C. T. Lauch, V. Vardaniar, L. Menchunor and P. T. Brown in a publication titled Continuous Real-Time Detection of Molecular Fluorine (F2) Emitted As a By-Product of CVD and Etch Processes, Semicon Southwest, Austin, Tex. October 2000, disclose a fluorine chemical sensor (FCS). In a fluorine chemical sensor, F2 oxidizes an organic substrate which then emits chemiluminescence to be detected by a photo multiplier tube (PMT). A fluorine chemical sensor cannot detect other homonuclear diatomic molecules such as Cl2, Br2, etc. because other halogen gases are not as oxidative as F2. On the other hand, other highly oxidizing species such as NF3 and O3 in the effluent stream may also react with the organic substrate and be falsely registered as F2. A fluorine chemical sensor response to F2 concentration is nonlinear. In addition to F2 concentration, a fluorine chemical sensor signal strength also depends on the mass flux to sensor surface area ratio. Therefore, effluent measurement must be made with the same mass flux as that of the calibration. This condition is sometimes difficult to satisfy due to changes in flow rates of slip stream sampling. A fluorine chemical sensor is suitable for low to moderate levels of F2 concentrations only. High concentrations of F2 may consume the organic substrate quickly and degrade its performance. Long term drift of the zero baseline and the signal response is also a concern when a fluorine chemical sensor is exposed to harsh corrosive gas streams.
In U.S. Pat. No. 6,023,065, the patentees disclose a method of using UV-Visible absorption spectroscopy to measure a bleaching agent, such as hydrogen peroxide (H2O2) and chlorine (Cl2), dissolved in pulp delignification waste water. The method involves measurements in an aqueous solution, hence conventional UV-Visible spectrometers and sampling devices can be employed. The gaseous effluent streams in the IC manufacturing processes present unique and different challenges to sampling and quantification of toxic and corrosive species.
In view of the problems with prior art devices and methods, it was desirable to develop a device to measure the quantities of halogens in semiconductor effluent streams that is less costly to transport, does not require frequent calibration, does not degrade expensive equipment, is readily interfaced to semiconductor process tools, and can be easily and quickly set up. It was also desirable to have a system that can detect all the homonuclear diatomic halogen gases simultaneously, and in real-time within an IC process effluent stream. It was further desirable that such a system should be rugged and stable, and suitable for a wide range of analyte concentrations. Furthermore, it was desirable to have a system that provides accurate endpoint for an IC manufacturing process such as remote plasma downstream chamber cleaning. Such an endpoint detector should be independent of the kind of materials deposited during the CVD processes.
According to one aspect, the present invention is a UV-Visible Light halogen gas analyzer system comprising a UV-Visible light source, an absorption cell, a fiber optic coupled spectrometer, and a computer, with mathematical relationships established between the light absorbance and the halogen concentrations. The system includes sampling for real-time on-line monitoring of halogen containing gases in IC manufacturing processing streams. In addition the system includes a slip-stream manifold for rapid and accurate sampling of process effluent streams.
A method and apparatus according to the invention provides accurate endpoint alternatives for IC manufacturing processes such as remote plasma downstream chamber cleaning.
Also the present invention pertains to an on-line sensor and method to monitor concentration of halogen species within a process effluent of a semiconductor manufacturing process to enable an operator to readily make changes to an operating parameter of the process such as, flow rates of reactants, electrode power, and process temperature.
Therefore, in one aspect, the present invention is a method for analyzing homonuclear diatomic halogen gases in a process effluent stream comprising the steps of: withdrawing a sample of the gas stream containing the halogen gases; introducing the sample of the gas stream into a sample cell having first and second ends containing windows to transmit UV-visible light through the sample; passing the UV-visible light through the sample cell and detecting light absorption by halogen gases in the cell; analyzing a selected absorption spectrum at selected wavelengths; and performing spectrum analysis to determine type and concentration of halogen gases present in the effluent stream.
In another aspect, the present invention is an apparatus for detecting the presence of a homonuclear diatomic halogen gases in a sample gas comprising in combination: a generally elongated sample cell having a first end and a second end with the first and second ends closed by UV-visible light transmitting windows/viewports; means to introduce a sample gas proximate a first end of the sample cell; means to remove the sample gas proximate the second end of the sample cell; means to pass a beam of UV-visible light through the cell from the first end of the cell to the second end of the cell; means proximate the second end of the cell to collect the UV-visible light and conduct the UV-visible light to a spectrometer to determine light intensities at differing wavelengths exiting the cell; and means to use the measured UV-visible light intensities to determine the presence and quantity of halogen gas in said sample gas.
In yet another aspect, the present invention is a method for detecting the endpoint of a clean operation wherein a halogen gas is used to clean contaminants from internal surfaces of a semiconductor process tool comprising the steps of: continuously withdrawing samples of an effluent gas stream from the semiconductor process tool; introducing the samples of the effluent gas into a sample cell having a first and second ends containing windows to transmit UV-visible light through the sample; passing the UV-visible light through the sample cell and collecting light passing through the cell; analyzing the light passing through the cell for absortion spectrums at wavelengths indicating the presence of the halogen gas in the sample of effluent gas; and using the spectrum to determine when a large increase in the amount of the halogen gas is present in a sample of effluent gas thereby indicating the tool is clean.
In a further aspect, the present invention is a method for controlling a semiconductor manufacturing process by monitoring the concentration of homonuclear diatomic halogen gases in a process gas stream comprising the steps of: withdrawing a sample of the gas stream containing said halogen gases; introducing said sample of the gas stream into a sample cell having first and second ends containing windows to transmit ultra violet and visible light through the sample; passing the light through the sample cell and detecting light absorption by halogen gases in the cell; analyzing a selected absorption spectrum at selected wavelengths; performing spectrum analysis to determine type and concentration of halogen gases present in the effluent stream; and using data from spectrum absortion to control the operating parameters of the process.